1. Field
Embodiments described herein relate generally to a nonvolatile memory device.
2. Description of the Related Art
A resistive random access memory (ReRAM) attracts attention as a nonvolatile memory device of the next generation because the resistive random access memory is hardly affected by downsizing and the capacity thereof can be increased. The resistive random access memory stores, in a nonvolatile manner, resistance information of an electrically-rewritable variable resistance element, for example, a high resistance state and a low resistance state. In such a resistive random access memory, for example, variable resistance memory cells are arranged in an array shape in crossing positions of a plurality of bit lines extending in parallel to one another in a first direction and a plurality of word lines extending in parallel to one another in a second direction perpendicular to the first direction. In each of the variable resistance memory cells, a storage layer in the variable resistance element, electrode layers of platinum (Pt), titanium nitride (TiN), or the like provided on and under the storage layer, and a rectifying element such as a Schottky diode, a PN junction diode, or a PIN diode are connected in series (see, for example, Japanese Patent Application Laid-Open No. 2009-157982).
Examples of a material of the storage layer include metal oxides such as NiO, V2O5, ZnO, Nb2O5, TiO2, WO3, CoO, ZrO2, and HfO2, a high resistance state and a low resistance state of which can be switched according to the control of a voltage value and an application time. Usually, the variable resistance element has high resistance in an initial state. Therefore, first, processing called forming for feeding, while adjusting voltage, electric current and forming a filament (a current path) in nanometer order in the variable resistance element to reduce the resistance is performed. Thereafter, reset processing for applying voltage to the variable resistance element having the reduced resistance to feed electric current and increase the resistance and set processing for applying voltage to the variable resistance element having the high resistance to feed electric current and reduce the resistance are performed to create two states of high resistance and low resistance. Resistance information is stored in the variable resistance element to cause the variable resistance element to function as a memory (see, for example, Japanese Patent Application Laid-Open No. 2008-227267).
However, during the forming or the set, when voltage is applied to the variable resistance element, resistance suddenly falls at certain voltage. Therefore, it is likely that large electric current flows to the variable resistance element and breaks the variable resistance element. When the resistance of the variable resistance element set during the set is too low, large reset current flows to the variable resistance element during the reset. A driving circuit element and a protection circuit element are broken by the current.